Steel is heated iron that has had carbon added to it to make it a stronger and harder material. Steel is prone to warping from excessive heat. The metal expands and contracts more than iron because of its lower coefficient of friction.
If the brake material is kept in temperatures that are within its normal operating range, iron is less prone to galling and irregular wear from friction. Cast iron rotors wear better. They also have more internal damping in them which reduces squealing sounds and does not warp. Iron rotors rust easier than steel rotors. They are also more brittle than steel and should be inspected for cracks frequently.
Iron brake rotors provide more braking power than steel rotors. They also have better brake force modulation than their steel counterparts. Iron rotors have better heat management and will not warp as easily as stainless steel rotors. However, steel rotors do not develop hard spots when they overheat as iron rotors do. In addition, steel is stronger than iron and is less likely to crack.